1. Field of the Invention
The present invention relates to a method for forming through holes on a work piece by use of laser beam.
2. Related Background Art
Conventionally, as a method for forming through holes, the machining that uses a drill or the like has been mainly practiced. However, with a method of the kind, it is difficult to process a hole of a diameter as small as xcfx86100 xcexcm or less, for example. Also, in recent years, along with the development of the high performance electronic equipment, it has been required to arrange its wiring in higher density. As a result, there is a need increasingly for processing fine holes at small pitches, such as a printed-circuit board which is a typical example of those requiring a larger amount of drilling operation.
As one of the processing methods to meet such requirement, there is a method for processing the sample locally by the coherent beam irradiated onto the work piece through the mask provided with openings which are partly arranged. For example, as disclosed in the specification of Japanese Patent Laid-Open Application No. 60-13449, when through holes are processed on a printed-circuit board where a metallic layer is bonded on the surface of an organic substrate, such as polyimide, polyester, glass epoxy, it is arranged that, at first the metallic layer on the surface is selectively etched into the pattern to be processed. Then, with this metallic layer as a mask, the coherent laser beam is irradiated onto the substrate to process the through holes. Also, as disclosed in the specification of Japanese Patent Laid-Open Application No. 61-48582, there is a fine processing method using the etching and the irradiation of the coherent laser beam in combination. In this method, photoresist film is formed by the same pattern in the same position on both faces of a work piece, and then, the etching process is suspended before the hole is penetrated. Then, after the resist film is removed, the total number or a given number of bridges thus formed are removed by use of the coherent laser beam that runs along the same locus as the photoresist pattern.
However, when a work piece is processed by use of the coherent laser beam by the application of this technique, there is a problem that only the hole whose leading end may become narrower can be formed, but no others. This is because the converged coherent laser beam is caused to irradiate the inclined faces due to the taper angle created when the laser process is executed. Then, conceivably, as compared with the energy concentration of the coherent laser beam irradiated to the flat surface, the energy of the irradiated coherent laser beam is attenuated in this event to be less than the limit of the energy concentration (the threshold value).
Since each of the through holes is configured to be narrower at its leading end (the so-called tapered configuration), the difference becomes greater between the diameter of the opening of the through hole on the side where the laser beam is incident upon (the entrance side), and that of the exit side inevitably.
For example, if the printed-circuit board is provided with conductive layers above and below the insulation layer, which are electrically connected themselves with each other through the conductive substance filled in the through hole formed on the insulation layer, it is preferable to make the area larger for each opening on the edges of the through hole. In this case, if the difference is great between the diameters of the openings on the edges of the through hole as described earlier, it may be difficult to secure the sufficient diameter of the opening on the exit side in some cases. If it should be attempted to secure the sufficient diameter of the opening on the exit side, the diameter of the opening on the entrance side should be made larger than actually needed. Thus, the structure becomes improper for the formation of the through holes which should be arranged in higher density.
Also, if through holes of the kind should be adopted for the discharge ports of an ink jet head, the thickness of the exit edge of each through hole (discharge port) becomes thinner locally depending on the taper angles. Then, there is a fear that the exit end of each through hole (discharge port) is chipped off due to the repeated cleaning by use of a blade or the like.
The present invention is designed in consideration of the technical problems discussed above. It is an object of the invention to provide a method for forming through holes which makes it easier to form each of them with a small difference between the opening diameters by the utilization of coherent laser beam that reflects from a work piece while in the laser ablation.
The inventors hereof have given attention to the reflected beam created from the work object in the ablation processing when the laser beam is projected to the work object through the photomask for the performance of the ablation processing in order to achieve the objectives of the present invention. Then, the method of the invention for forming through holes, which has laser beam as the light source to project the laser beam to the work object using an optical system through a photomask for the formation of through holes on the work object by ablation processing, comprises the steps of increasing the concentration of the optical processing energy contributing to the process using the reflected beam created from the work object in the laser ablation processing; and forming each of through holes having the configuration enabling the narrower end to be changed to the wider end in the incident direction of laser beam.
Also, the method of the invention for forming through holes is provided with laser beam as the light source to project the laser beam to the work object using an optical system through a photomask for the formation of through holes on the work object by ablation processing. For this method, it is arranged that the photomask comprises a light shielding portion to form the unexposed portion in the interior of the through hole formed on the work object, and a light transmitting portion surrounding the light shielding portion to form each of the through holes.
With the structure thus arranged, it becomes possible to increase the energy concentration that contributes to the process as compared with the usual ablation processing, because the reflected beam created in the ablation processing can be utilized again for the optical processing. As a result, each of the through holes can be formed easily in the configuration in which the narrower end changes to the wider end in the incident direction of laser beam, which cannot be easily processed by the application of the usual ablation processing. Then, in accordance with the method of the present invention, the difference is made smaller between the opening diameters, hence making it possible to apply the method preferably for the formation of through holes which are arranged in higher density. Also, the section of the edge portion formed inner side of the end portion of the through hole does not present any acute angle for the through holes of the minimum diameter. As a result, if the through holes thus formed are applied to the discharge ports of an ink jet head described earlier, it becomes possible to reduce the chip off of the edge portion thereof significantly.
Further, with the structure of the photomask thus arranged, the amount of laser irradiation to the work piece becomes smaller than the conventional method. As a result, it becomes possible to significantly reduce the expansion of the work piece due to heat generated in the laser processing. Also, with the unprocessed portion that resides in the interior of each through hole, it is possible to form each of the through holes having a lesser amount of flash.